This application is based on French Patent Application No. 01 07 464 filed Jun. 7, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to a method of balancing a modular electrical storage battery comprising rechargeable cells connected in series. It also relates to a battery management system adapted to be incorporated into a battery charging and balancing facility to enable use of the method.
The method according to the invention is more particularly intended for use in applications where a battery is liable to be subjected to discontinuous charging, for example because the charger does not operate continuously or because the battery is not always electrically connected to a charger.
2. Description of the Prior Art
The method and the battery management system are in particular intended to be used with batteries which comprise a relatively large number of cells, because of the quantity of energy to be stored. As is well known to the person skilled in the art, the cells of the same battery, which are usually identical, can have different characteristics and consequently slightly different performance. The difference in performance between cells of the same battery can significantly increase over time and in the course of successive charging and discharging. This can degrade the cells of a battery and lead to imperfect operation of the load that the battery supplies with power.
The same charging or discharging current generally flows through the series-connected cells of a battery. However, balancing a battery sometimes requires different currents to flow through its various cells. Balancing a battery by discharging one cell by means of an individual shunt connected in parallel with it if the cell in question is overcharged is known to the person skilled in the art. The shunt includes a resistor for dissipating energy, for example. A shunt or bypass current then flows through the shunt and reduces the current flowing in the cell, or even shunts all of the current. The balancing capacity Q for a cell is proportional to the shunt current I and to the time for which the shunt is applied. It is therefore possible to obtain the same cell balancing effect with a high shunt current that flows for a relatively short time or a low shunt current that flows for a relatively long time. Historically, the first solution to be adopted was that using a high shunt current, as this enables balancing to be achieves in a short time, which is important if the time available for balancing is short. Allowing a shunt current to flow at one battery cell only from the time at which a particular activation threshold voltage is reached at the terminals of that cell is known to the person skilled in the art. The time necessary for this threshold voltage to be reached is subtracted from the time available for balancing the battery. The shorter the time available for balancing, the higher the shunt currents for the most highly charged battery cells.
One method known to the person skilled in the art uses a battery supervised by centralized logic for controlling the shunts as a function of voltages measured at the terminals of the individual cells with which the shunts are associated. The voltages present at the terminals of the battery cells are collated centrally to determine the highest voltage. The centralized logic activates the shunt assigned to the cell with this highest voltage at its terminals. This method performs balancing with low shunt currents, and the balancing time can be long. On the other hand, it has the drawback of necessitating relatively complex centralization, and this implies considerable software development. This is only a minor inconvenience in the case of high-voltage batteries, but there are many applications with only a few cells per battery where this kind of solution is not desirable, in particular for reasons of cost, which can be considered to be practically constant regardless of the number of cells.
Using the battery as a buffer between a charger and a load is also known to the person skilled in the art, in applications where a load is supplied with power continuously, as is conventionally the case in the field of telecommunications, in which case the battery is kept charged at a constant voltage, provided that the charger is supplied with power. In this case it is possible to carry out balancing cell by cell and starting smoothly from a particular threshold voltage per cell, which voltage increases as the measured voltage for each cell increases. A high shunt current represents the end of charging of the battery cell concerned, and is detected in order to instruct the charger to reduce the regulation voltage that it is supplying to the battery. The charges in the battery cells tend to balance out, since the shunt currents are a function of the respective voltages measured at the terminals of the various cells. In some applications, and in telecommunications applications in particular, the charger is supplied with power most of the time, so that balancing can be carried out over time with low shunt currents, and, because the battery is on virtually permanent charge, it is not discharged by any shunt currents that may be established at its cells. However, the method is not particularly suitable if quasi-continuous charging of a battery by a charger cannot be systematically achieved. In this case, the same drawbacks are encountered as for the first prior art solution, which relate to the fact that the time available for balancing can be short, the shunt currents must then be high, and the battery is partially discharged during the balancing phase.
The invention therefore proposes a method of balancing electrical batteries comprising a plurality of cells connected in series and which are charged by means of a charger under the control of a battery management system which associates an individual current shunt circuit in parallel with each cell and is activated if the voltage at the terminals of the cell exceeds a minimum balancing threshold value common to all the cells.
According to one feature of the invention, the method reduces the discharging of the battery cells during balancing when the battery is no longer supplied with power by a charger.
According to one feature of the invention, a higher balancing threshold voltage is substituted for the minimum balancing threshold voltage from the time at which the battery has not been supplied with power by the charger for a time greater than a particular time-delay. This prevents inappropriate discharging of the battery when it is not on charge whilst allowing balancing of the most highly charged cells.
The method according to the invention applies more particularly in the context of a battery management system which includes a control daisychain for continuously supplying a binary signal consisting of a current flowing between the end terminals of the battery through a series of transmission control gates each of which is assigned to a respective cell. Each gate is controlled so that it interrupts the flow of current in the control daisychain if the voltage at the terminals of the cell to which it is assigned exceeds a maximum voltage threshold value, any interruption of the current causing a switching control signal to be sent to the charger via a common control interface.
In the method according to the invention, a higher balancing threshold voltage is substituted for the minimum balancing threshold voltage in response to the interruption of the current flowing in the control daisychain and after a time exceeding a particular time-delay. This is a particularly economical way to implement the method according to the invention.
In one particular embodiment of the invention, the time for which the current flows in the control daisychain is measured to control incrementing of the voltage supplied to the battery by the charger by predetermined steps, and likewise the time that has elapsed since the interruption of the current is measured to control decrementing the voltage supplied to the battery by the charger by predetermined steps.
The invention also proposes a battery management system for a facility for charging and balancing a battery comprising rechargeable cells connected in series and adapted to be supplied with power by a charger adapted to supply at least one constant regulation voltage for charging the battery.
The battery management system includes:
an individual interface for each battery cell, the interface including:
a first voltage measuring circuit connected to the terminals of the cell to command a current shunt connected in parallel with it to shunt at least a portion of the charging current that is supplied to it if the voltage measured at the terminals of the cell rises above a balancing minimum threshold value;
a second voltage measuring circuit for determining if a maximum threshold value is exceeded at the terminals of the cell from a voltage measured across a measuring resistor in series with the shunt of the interface between the terminals of the cell; and
a transmission control AND gate combining a binary signal received by the interface at an input terminal and a binary signal produced by the second measuring circuit of the interface to produce a resultant binary signal at an output terminal of the interface and corresponding to a current if the received signal is a current and the voltage measured by the second measuring circuit is less than the maximum threshold value; and
a common interface at the output of a control daisychain and connected to one end terminal of the battery via series-connected transmission gates of the individual interfaces to operate on the charger in the event of interruption of the resulting current sent via the transmission control gates of the individual interfaces if at least one voltage measured at the terminals of the cells of the battery exceeds the maximum threshold value and the control daisychain is interrupted for this reason or any other reason.
The facility includes a switching unit, between an end terminal of the battery and an input terminal of the individual interface situated at the head of the control daisychain and assigned to the cell connected to the end terminal, for interrupting the resulting current that flows through the control daisychain, and a first measuring circuit in each individual interface which includes means for substituting a balancing threshold value higher than the minimum balancing threshold value if the current flowing between the battery terminals via the control daisychain is interrupted.
According to the invention, the battery management system also includes time-delay means associated with the switching member to delay the interruption of the current flowing through the control daisychain for a predetermined time after the charger stops supplying charge to the battery.
According to the invention, the common control interface includes and controls a counter for commanding stepwise switching of the voltage supplied by the charger to the battery, which voltage is incremented in steps if the common interface receives the current flowing in the control daisychain, and decremented in steps, preferably at a faster rate, if the current flowing in the control daisychain is interrupted.
The invention, its features and its advantages are explained in the following description, with reference to the accompanying drawings.